Most of the energy of the world can be produced by oil, coal, natural gas or nuclear power. All these production methods have their specific issues as far as, for example, availability and friendliness to environment are concerned. As far as the environment is concerned, for example, oil and coal can cause pollution when they are combusted. The issue with nuclear power is, at least, storage of used fuel.
Because of the environmental issues, new energy sources, more environmentally friendly and, for example, having a better efficiency than the above-mentioned energy sources, have been developed.
Fuel cells, by which energy of fuel, for example biogas, can be directly converted to electricity via a chemical reaction in an environmentally friendly process, are promising future energy conversion devices.
A fuel cell, as shown FIG. 1, can include an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. In solid oxide fuel cells (SOFCs) oxygen 106 can be fed to the cathode side 102 and it can be reduced to a negative oxygen ion by receiving electrons from the cathode. The negative oxygen ion goes through the electrolyte material 104 to the anode side 100 where it reacts with fuel 108 producing water and carbon dioxide (CO2). Between anode 100 and cathode 102 can be an external electric circuit 111 including a load 110 for the fuel cell.
FIG. 2 shows a SOFC device, which is an example of a high temperature fuel cell device. SOFC devices can utilize as fuel for example natural gas, biogas, methanol or other compounds containing hydrocarbons. The SOFC device in FIG. 2 can include more than one, for example, plural fuel cells in stack formation 103 (SOFC stack). Each fuel cell can include anode 100 and cathode 102 structure as presented in FIG. 1. Part of the used fuel can be recirculated in feedback arrangement 109 through each anode. The SOFC device in FIG. 2 also can include fuel heat exchanger 105 and reformer 107. Several heat exchangers, for example, can be used for controlling thermal conditions at different locations in a fuel cell process. Reformer 107 is a device that converts the fuel, such as for example, natural gas to a composition suitable for fuel cells, for example to a composition containing hydrogen and methane, carbon dioxide, carbon monoxide and inert gases. In each SOFC device, a reformer is not necessary.
By using measurement means 115 (such as fuel flow meter, current meter and temperature meter) measurements can be carried out for the operation of the SOFC device. Part of the gas used at anodes 100 can be recirculated through anodes in feedback arrangement 109 and the other part of the gas is exhausted 114 from the anodes 100.
A solid oxide fuel cell (SOFC) device can be an electrochemical conversion device that produces electricity directly from oxidizing fuel. An SOFC device can include high efficiencies, long-term stability, low emissions, and cost. One issue with an SOFC device can be the high operating temperature, which results in long start up and shutdown times and mechanical and chemical compatibility issues.
Natural gases such as methane and gases containing higher carbon compounds can be used as fuels in SOFCs, which gases, however, have to be preprocessed before feeding to the fuel cells to prevent coking, for example, formation of harmful carbon compounds such as, for example coke, fly dust, tar, carbonate and carbide compounds. These different forms of carbon can be in this context called as a general term, harmful carbon compounds. Hydrocarbons can go through a thermal or catalytic decomposition in the formation of harmful carbon compounds. The produced compound can adhere to the surfaces of the fuel cell device and adsorbs on catalysts, such as nickel particles. The harmful carbon compound produced in the coking can coat some of the active surface of the fuel cell device, thus significantly deteriorating the reactivity of the fuel cell process. The harmful carbon compounds can even completely block the fuel passage.
Preventing formation of harmful carbon compounds can be important for ensuring a relatively long service life for the fuel cells. The prevention of formation of harmful carbon compounds can also save catalysts that are the substances (nickel, platinum, etc.) used in fuel cells for accelerating chemical reactions. Gas pre-processing uses water, which can be supplied to the fuel cell device. The water produced in combining the oxygen ion and the fuel, for example, the gas on the anode 100 side, can be used in the pre-processing of the gas.
The anode electrode of solid oxide fuel cell (SOFC) can contain amounts of nickel that can be vulnerable to form nickel oxide if the atmosphere is not reducing. If nickel oxide formation is severe, the morphology of electrode can be changed irreversibly causing significant loss of electrochemical activity or even break down of cells. Hence, SOFC systems purge gas, for example, safety gas, containing reductive agents (such as hydrogen diluted with inert such as nitrogen) during the start-up and shut-down in order to prevent the fuel cell's anode electrodes from oxidation. In practical systems, the amount of purge gas has to be minimized because an extensive amount of, for example pressurized gas containing hydrogen, can be expensive and problematic as space-requiring components. Purge gases are not necessarily elemental and they can be compound gases.
Processing of CPOx (Catalytic Partial Oxidation) in fuel cell systems can produce carbon monoxide (CO) and hydrogen (H2). Fuel cell system start-up or shutdown operation can include sufficient steam and hydrogen production, where CO production in large amounts can be harmful. Using higher air, for example, oxygen amounts for more complete oxidation can produce too much heat making temperature raise excessive in the start-up situation or cooling process too slow in the shutdown situation.
CPOx (Catalytic Partial Oxidation) can produce carbon monoxide (CO) and hydrogen (H2). This gas mixture can be used for various chemical industry purposes, and the operating temperature of CPOx can be above 700° C. The known product gas product can be unsuitable for fuel cells due to coke formation in the system heating/operating temperatures. Start-up or shutdown gas can include sufficient steam and hydrogen production, whereas CO production in larger amounts can be harmful. Using higher air, for example, oxygen amounts for more complete oxidation, can produce too much heat making the temperature raise excessive in regards to normal SOFC operating conditions, thermal management, thermal stresses and material selection.
U.S. Patent Publication No. 2011/159386 A1 discloses a process for starting up a fuel cell system, which has a fuel cell with a cathode side and an anode side, a reformer and an auxiliary burner. Fuel cell air can be preheated with the auxiliary burner and fed to the cathode side of the fuel cell. Residual gas is circulated from the anode side of the fuel cell to the reformer and from the reformer to the anode side. Air fed to the anode side is stopped in order to remove oxygen from the anode side recirculation.
U.S. Patent Publication No 2006/093879 A1 discloses a procedure for starting up a fuel cell system having an anode exhaust recycle loop. The fuel cell system can be disconnected from its primary load and has air in both its cathode side and anode side. Gas from recirculation of the anode side flow is exhausted and only a small limited flow of fuel is provided into the anode side recirculation. Hydrogen and oxygen in the fuel and air mixture can be catalytically reacted as they recirculate in the anode side until substantially no oxygen remains in the recycle loop, and then the fuel flow rate into the anode side flow can be increased to normal operating levels and thereafter connecting the primary load across the cell.
U.S. Patent Publication No. 2006/093879 A1 discloses a reforming stage of the fuel cell system, wherein oxygen is removed from the anode side. Hydrogen and water steam can be fed to the anode side instead of production of them.
U.S. Patent Publication No. 2002/102443 A1 discloses a procedure for shutting down a fuel cell system having an anode exhaust recycle loop. A portion of the anode side flow exhaust is recirculated through the anode side in a recycle loop during operation. The fuel cell system is shut down by disconnecting the primary load from the external circuit and thereafter stopping the flow of fresh hydrogen containing fuel into the anode side flow and catalytically reacting hydrogen in the anode side recirculation by recirculating such gases within the anode recycle loop into contact with a catalyst until substantially all the hydrogen is removed.
U.S. Patent Publication No. 2002/102443 A1 discloses a similar but reversed method to that presented in U.S. Patent Publication No. 2006/093879 A1.
EP 1571726 A1 and EP 1998398 A2 disclose known systems.